1. Field of the Invention
This invention relates to heat sinks, and particularly to heat pipes.
2. Description of the Related Art
Semiconductor systems such as laser diode arrays, compact motor controllers and high power density electronics increasingly require high-performance heat sinks that typically rely on heat pipe technology to improve their performance. Rotating and revolving heat pipes, micro-heat pipes and variable conductant heat pipes may be used to provide effective conductivity higher than that provided by pure metallic heat sinks. Typical heat pipes that use a two-phase working fluid in an enclosed system consist of a container, a mono-dispersed or bi-dispersed wicking structure disposed on the inside surfaces of the container, and a working fluid. Prior to use, the wick is saturated with the working liquid. When a heat source is applied to one side of the heat pipe (the “contact surface”), the working fluid is heated and a portion of the working fluid in an evaporator region within the heat pipe adjacent the contact surface is vaporized. The vapor is communicated through a vapor space in the heat pipe to a condenser region for condensation and then pumped back towards the contact region using capillary pressure created by the wicking structure. The effective heat conductivity of the vapor space in a vapor chamber can be as high as one hundred times that of solid copper. The wicking structure provides the transport path by which the working fluid is recirculated from the condenser side of the vapor chamber to the evaporator side adjacent the heat source and also facilitates even distribution of the working fluid adjacent the heat source. The critical limiting factors for a heat pipe's maximum heat flux capability are the capillary limit and the boiling limit of the evaporator wick structure. The capillary limit is a parameter that represents the ability of a wick structure to deliver a certain amount of liquid over a set distance and the boiling limit indicates the maximum capacity before vapor is generated at the hot spots blankets the contact surfaces and causes the surface temperature of the heat pipe to increase rapidly.
Two countervailing design considerations dominate the design of the evaporator wicking structure: Liquid transport capability and vapor transport capability. A wicking structure consisting of sintered metallic granules is beneficial to create capillary forces that pump water towards the evaporator region during steady-state operation. However, the granular structure itself obstructs transport of vapor from the evaporator region to the condenser region. Unfortunately, conventional heat pipes can typically tolerate heat fluxes less than 80 W/cm2. This heat flux capacity is too low for high power density electronics that may generate hot spots with local heat fluxes on the order of 100-1000 W/cm2. The heat flux capacity of a heat pipe is mainly determined by the evaporator wick structures. Carbon nano tubes grown in a “forest” structure or grown to form microchannel fins have also been explored for use as evaporator wicking structures. In the case of an evaporator wicking structure formed of microchannel nano tube fins, inner-surfaces between microchannel fins have also been treated with nano tubes to further increase the thermal exchange rate.
A need still exists for a heat pipe with increased capillary pumping pressure with better vapor transport to the condenser to enable higher local heat fluxes.